1. Field of the Invention
The present invention relates to a sheet transfer apparatus for transferring a sheet to a prescribed processing position, and an image reading apparatus mounted on the sheet transfer apparatus for optically reading an image of the sheet transferred to a prescribed processing position to output read image data, and more specifically to the structure for detecting a transfer position of the sheet to be transferred to a prescribed position.
2. Description of the Related Art
Conventionally, in the sheet transfer apparatus mounted on the image reading apparatus in a copying machine, a printer or the like, the sheet is sometimes obliquely moved to arise the lateral deviation (or dislocation). When the lateral deviation of the sheet arises, an image formed on the sheet is inclined or deviated. Therefore, the procedure has been generally carried out in which the lateral deviation is detected by a sensor, and the deviation of the image is corrected on the basis of the detected result.
Further, for detecting the lateral deviation amount and correcting a write position of an image according to the lateral deviation amount, light receiving elements having many pixels such as a line sensor, an image sensor or the like represented by a photodiode array, a CCD sensor, a CMOS sensor, and a CIS sensor are used. Means for detecting the lateral deviation position by ON/OFF of every pixel are known by, for example, Japanese Patent Laid-Open No. 6 (1994)-92512 Publication, Japanese Patent Laid-Open No. 7 (1995)-187449 Publication, Japanese Patent Laid-Open No. 2002-292960 Publication, and the like. In this case, on the light emitting side, a light emitting area to the same degree as the length on the light receiving side is necessary, and therefore, parallel light is formed by using a LED array, a diffusion plate, or a lens and a slit.
However, the CCD sensor, the CMOS sensor and the like are expensive, and a circuit exclusively used for signal processing is necessary. Further, in the photodiode array, a number of minute chips are arranged in a row, and fundamentally, it has the structure similar to that of the CCD sensor or the like, resulting in increasing the cost.
Further, when the LED array, the diffusion plate, the lens or the like are used on the light emitting side in order to form the light emitting area (parallel light) to the same degree as the length of the light emitting portion, not only a large space is necessary but also the price becomes high. It is therefore contemplated that the light emitting portion is made small as compared with the light receiving portion. However, if the light emitting portion is small as compared with the light receiving portion, light from the light emitting portion to the light receiving portion is widened, and therefore where the edge of the sheet is in the center of the light source, an accurate position thereof can be obtained, whereas where the edge of the sheet is positioned to one side laterally from the center of the light source, the edge of the sheet positioned to one side laterally from the actual position is detected. The sheet and the light receiving portion have to be positioned as close as possible, in which case, however, there arises a new problem that the freedom of design is small.
Further, in the general sheet transfer apparatus, sheets of various sizes are being handled. As the systems for transferring the sheets of various sizes, there have been already known a one side reference transfer system for adjusting one end in the width direction in the sheets of all sizes to a reference position provided on one side on a feed tray to transfer the sheets, and a center reference transfer system for adjusting the center in the width direction in sheets of all sizes to the center on a feed tray.
In the one side reference transfer system, if the lateral deviation amount detection sensor is arranged in a relatively short range of a transfer channel corresponding to a reference position provided on one side on the feed tray, the detection of sheets of all sizes becomes enabled. However, in the center reference transfer system, since the transfer position of the end is different depending on the sheets of respective sizes, at least a light receiving element of the lateral deviation amount detection sensor need to have a length corresponding to a position of the end in the width direction in each sheet. Therefore, the length of the light receiving element becomes long, but the resolving power of an output voltage from a current-voltage conversion circuit is constant, and therefore the detection accuracy of the lateral deviation amount of the sheet lowers as compared with the case where the light receiving element is short (case of the lateral deviation amount detection sensor in the one side reference transfer system). Further, if approximately uniform light is not incident on the light receiving element, the detection accuracy lowers, and therefore as previously mentioned, a light emitting area (parallel light) of the same degree as the length of the light receiving element has to be formed on the light emitting side by an LED array, a diffusion plate or an lens. Thus, if the light receiving element is longer, not only a larger space is necessary but also the price becomes high.
The present invention has been accomplished paying attention to the aforementioned circumstances, and an object of the invention is to provide an apparatus in which the position deviation amount in the width direction of a transfer sheet is detected accurately and at less cost, and even if the size in the width direction of the transfer sheet is different, the position deviation amount in the width direction of the transfer sheet can be detected with high accuracy without bringing forth an increase of the number of parts and complication of the construction.